1. Field of the Invention
The present invention relates to an image reading apparatus.
2. Description of the Related Art
Conventional image reading apparatuses have been disclosed in, for example, Japanese Patent Application Laid-open Nos. 2005-167854 and 2002-300394.
Japanese Patent Application Laid-open No. 2005-167854 discloses a technique for improving productivity of image reading of an original by performing shading correction most efficiently and correctly depending on situation and by reducing a reading interval between originals without deteriorating the quality of a read image. Specifically, if a time Td between originals, required for performing a sheet-through operation without stopping, is longer than a shading operating time Ts of a document feeder (DF), successive reading is performed in a nonstop operation. When a memory permission notification is notified from a controller after the sheet-through operation is decelerated, the sheet-through operation is performed after the memory permission notification is received. When size reduction of an original is specified in reading the original, the DF is caused to start the sheet-through operation when a time T (=Tds−Td) has passed after the memory permission request is received from the controller, where Tds is a time from receiving of the memory permission notification to finishing of the shading correction by the DF.
Japanese Patent Application Laid-open No. 2002-300394 teaches a shading correction function facilitating high-speed reading and minimizing degradation of scan per minute (SPM) to the utmost. Specifically, when a sheet-through automatic document feeder successively feeds originals one by one, a shading operation and a light-quantity measuring operation is performed with respect to the first original. The shading operation includes stopping the sheet-through operation, scanning a white plate by using a scanner unit to acquire white data. Shading correction data is generated from the white data and the shading correction data is used to perform shading correction of image data acquired from the originals. The light-quantity measuring operation includes measuring the light quantity of an exposure lamp that illuminates originals with the scanner unit. The light quantity data of the exposure lamp measured in the light-quantity measuring operation corresponding to the first original is saved as a reference value.
Even when the setting is such that the shading operation is not to be performed with respect to the second and subsequent originals, the light-quantity measuring operation is still performed with respect to each subsequent original (i.e., the second original to the last original) thereby acquiring subsequent light quantity data. If the difference between the reference data and the subsequent light quantity data is out of a preset range, it means that the conditions have changed whereby the reference value is no more appropriate. In this situation, the reference value is updated with the latest subsequent light quantity data.
FIG. 7 is a flowchart of the process of intermittent shading correction performed by the image reading apparatus disclosed in Japanese Patent Application Laid-open No. 2002-300394. The scanner unit performs a density measuring operation in an interval between sheets of paper (Step S101). The density measuring operation includes measuring a density of a member that is provided on the original feeding side. “Interval between sheets of paper” in this specification means a period between a time point at which the trailing edge of a preceding original leaves a scanning position and a time point at which the leading edge of a subsequent original reaches the scanning position. In other words, in the density measuring operation the scanner unit measures the density of the member after the trailing edge of the first original has passed a scanning position and before a leading edge of the second original reaches the scanning position. If the preceding original is the first original, then the measured density is saved as a reference value (Steps S102 and S103).
If the preceding original is an original other than the first original (Step S102), an amount of change is calculated based on the reference value and the density measured at Step S101 (Step S104). The amount of change is expressed in percent. An allowable range of the amount of change is previously determined. It is determined whether the calculated amount of change is within the allowable range (Step S105). If the calculated amount of change is within the allowable range, the process is terminated (YES at Step S105). On the other hand, if the calculated amount of change is out of the allowable range (NO at Step S105), it means that the current reference value is no more appropriate for the current conditions. In this situation, the reference value is updated (Step S106).
When updating the reference value, first, an instruction to stop the sheet-though operation is output (Step S107), so that feeding of originals is stopped. In this situation, the shading operation is performed to acquire new white data (Step S108). Upon completion of the shading operation (Step S109), the existing white data is corrected, or updated, with the new white data. Finally, an instruction to start the sheet-though operation is output (Step S110) so that feeding of originals is started, and the process is terminated. Thereafter, Steps S101 to S110 are repeated for all the originals to be read.
Japanese Patent Application Laid-open No. 2002-300394 proposes an image reading apparatus that includes an automatic document feeding mechanism capable of enhancing productivity by not executing the shading operation for each original, but by reducing intervals of the shading operation to thereby reduce a waiting time for the shading operation. It is suggested that the productivity can be enhanced by optimizing an interval between originals using arbitrary means based on whether or not the shading operation is to be executed. Specifically, as understood from the flowchart in FIG. 7, sheet-through operation (original feeding) is explicitly stopped (Step S107) before beginning the shading operation, and the sheet-through operation is restarted after the shading operation is completed (Step S110). Meanwhile, when the shading operation is not to be executed (YES at Step S105), i.e., when the calculated amount of change is within the allowable range, the timing of starting the sheet-through operation is not certain. Various drawbacks occur if this timing is not correct. Therefore, the sheet-through operation cannot always be performed in a nonstop manner, and the productivity cannot thereby be maximally improved.
Specifically, as shown in a timing chart in FIG. 8, if the sheet-through operation is to be performed in a non-stop manner, the start of sheet-through operation needs to be triggered before a sheet-through deceleration period L1 starts. Otherwise, the sheet-through operation cannot be started at a timing T1 at which a sheet-through stop period L2 is over. However, Japanese Patent Application Laid-open No. 2002-300394 does not disclose about when and how to start the sheet-through operation when the shading operation is not to be executed, i.e., when the calculated amount of change is within the allowable range.
Meanwhile, Japanese Patent Application Laid-open No. 2005-167854 proposes an image reading apparatus capable of enhancing productivity by comparing a time interval between originals when the originals are fed nonstop using the automatic document feeder with a time required for the shading operation, and of feeding the originals nonstop when the shading time is shorter than the other. Therefore, because the shading time becomes substantially zero when the shading operation is not executed in the technology disclosed in Japanese Patent Application Laid-open No. 2002-300394, a nonstop operation determination mechanism described in Japanese Patent Application Laid-open Nos. 2005-167854 is used to feed the originals nonstop, which can achieve improvement of the productivity.
However, in the document feeder, when the trigger of the start of sheet-through operation is received before the sheet-through operation is decelerated, the sheet-through operation is performed nonstop, and when the trigger is received after the sheet-through operation is decelerated, the operation is performed with deceleration of the sheet-through operation and stop of the sheet-through operation. Therefore, to switch between operations so that the nonstop sheet-through operation is performed when the shading operation is not executed and the stop operation is performed when it is executed, it is necessary to provide a mechanism to explicitly switch to the sheet-through operation.